Easiest I've found is made by Omega and sold by Sonic Electronix.
Basically for $78 you buy the plug n play adaptor and plug it in to your computer under the dash and it will work with your factory key fob, just have to push the lock button three times to start it. They also make an extender key fob that will go up to 1500 feet. And if you really want to get fancy you can buy the OmegaLink along with a yearly fee of $50-$100 and you can control it all from your phone from anywhere in the world.
I've spent many hours trying to find the best option and this is what I beleive to be the best option without having to mess around with wires, etc.
planning to get one for the wife for xmas for our 2006 Commander.